MODULATION OF THE PROTON-TRANSFER EQUILIBRIUM OF THE ADDUCTS BETWEEN 2-HYDROXY-P-QUINONES AND 4-(N,N-DIMETHYL)AMINOPYRIDINE - A SEMIEMPIRICAL MO STUDY

AMI and PM3 semiempirical calculations were performed on the adducts b etween 2,5-dihydroxy-1, 4-benzoquinone (DHpQ) or 2-hydroxy-5-methyl-1, 4-benzoquinone (HMQ) and 4-(N,N-dimethyl)aminopyridine (DAPY). Such s ystems can be employed as a model for the active site of copper-contai ning amine oxidases. The present paper shows that the DHpQ-DAPY (or HM Q-DAPY) adduct can exist in two tautomeric forms, namely a hydrogen-bo nded (OH...N) and a proton transfer one (O-...HN+). According to semie mpirical calculations, the former is more stable in vacuum but, as an increasing number of proton-donor molecules are coordinated to the sys tem through hydrogen bonding, a progressive stabilisation of the latte r species is obtained. The proton-donors examined were, besides water, different alcohols, chloroform and methylene chloride. Their efficien cy in hydrogen bonding to the solute was compared and turned out to va ry linearly with the solvatochromic parameter alpha of Kamlet and Taft (M.J. Kamlet, R. Taft, J. C. S. Perkin II; (1979) 349). It was also s hown that, in general, the alpha-scale of solvent hydrogen-bond-donor ability linearly correlates with the AM] or PM3 calculated charge dens ity on the proton engaged in the hydrogen bond. For a limited number o f solvents (water and alcohols), alpha also linearly correlates with t he experimental gas-phase acidity. (C) 1998 Elsevier Science B.V.